Analysis: A Figure of Merit for Efficiency Roll-off in Thermally Activated Delayed Fluorescent Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) are a revolutionary self-emitting display technology that has been successfully commercialized in mobile phones and TVs. The injected charges form both singlet and triplet excitons, and for high efficiency it is important to enable dark triplets to emit light. Currently materials that harvest triplets by thermally activated delayed fluorescence (TADF) are a very active field of research as an alternative to phosphorescent emitters which frequently employ heavy metal atoms. Whilst excellent progress has been made, there is a severe decrease of efficiency as the drive current is increased, i.e. efficiency roll-off, in most TADF OLEDs. At present much of the literature suggests that efficiency roll-off should be reduced by minimising the energy difference between singlet and triplet excited states in order to maximise the rate of conversion of triplets to singlets via reverse intersystem crossing (kRISC). We analyse the efficiency roll-off in a wide range of TADF OLEDs and find that neither of these parameters fully accounts for the reported efficiency roll-off. By considering the dynamic equilibrium between singlets and triplets in TADF materials, we propose new figures of merit (FOMs) for efficiency roll-off and discuss their correlation with reported data of TADF OLEDs. Our new FOMs will guide the design and development of TADF-OLEDs with lower efficiency roll-off. This will also lead to improved efficiency of TADF OLEDs at realistic display operating conditions and help expand the use of TADF materials to applications that require high brightness, such as lighting, augmented reality, and lasing.